The present invention relates to photovoltaic cells (also known as solar cells) for producing electricity from sunlight, and for improved systems and methods therefor.
The invention has particular relevance for solar cells of the dye sensitized type, and is applicable to other types of solar cells, and devices such as screen displays and electronic or defrostable windows where a high current density of operation at minimal ohmic loss is advantageous. A further application is in solar thermal systems (e.g., solar water heaters) where heat and power may be provided from the same collector area/structure.
Dye-sensitized photovoltaic cells for producing electricity from sunlight have been disclosed by U.S. Pat. No. 5,350,644 to Graetzel, et al. U.S. Pat. No. 5,350,644 teaches a photovoltaic cell having a light-transmitting, electrically-conductive layer deposited on a glass plate or a transparent polymer sheet to which a series of titanium dioxide layers have been applied, in which at least the last titanium dioxide layer is doped with a metal ion that is selected from a divalent or trivalent metal.
Following U.S. Pat. No. 5,350,644, U.S. Pat. No. 6,069,313 to Kay teaches a plurality of series-connected cell elements arranged as separate parallel elongated stripes on a common electrically insulating transparent substrate. Each element includes a light facing anode, a counter-electrode or cathode, and an intermediate, light-sensitive electrically-insulating porous layer separating the anode from the cathode. The pores of the intermediate layer are at least partially filled with a liquid phase, ion-transferring electrolyte and a light sensitive dye. An additional current collecting layer of a transparent, electrically-conducting material is situated between the substrate (glass or transparent polymer) and each of the anode and cathode. The anode and cathode of a given cell provide a direct-current voltage when the anode is exposed to light and series assemblies of cells may readily be built up. The cathode of each succeeding element is connected with the intermediate conducting layer of the preceding anode element, over a gap separating the respective intermediate layers of these two elements.
The cells of the above-cited prior art (an example of which is provided in FIG. 1a) are much closer conceptually to battery cells than to conventional photovoltaic cells, since the charge generators are separated by an electrolyte and are not in direct contact. These cells have two electrodes separated by an electrolyte, with one electrode (the photoelectrode) facing the sun. Each electrode is supported on its own current collector, usually a sheet of conducting glass, which is optical glass coated on one side with a thin (about 0.5 microns) transparent layer usually based on electrically-conductive tin oxide, and the conducting glass sheets act as transparent walls of the dye cell.
A transparent polymer may be used in place of glass to support the tin oxide. The photoelectrode includes a transparent porous layer about 10 microns thick (in contact with the tin oxide layer) based on titania, having a nanocrystalline characteristic particle size of 10-50 nm, applied by baking onto the conductive glass or transparent polymer, and impregnated with a special dye. The baked-on titania layer is applied in dispersion form by doctor blading, rolling, spraying, painting, gravure printing, screen printing or printing, but the baking step in some experimental procedures is in excess of 400° C., requiring the use of conducting glass rather than plastic for supporting the titania layer. Other processing procedures for the titania layer are feasible, such as reduced temperature baking, or pressing, usually with some sacrifice in efficiency.
The other electrode (the counter electrode) includes a thin layer of catalyst (usually containing a few micrograms of platinum per square cm) on its respective sheet of tin-oxide coated conductive glass or transparent plastic. The electrolyte in the cell is usually an organic solvent with a dissolved redox species. The electrolyte is typically acetonitrile or a higher molecular weight nitrile, with the redox species being dissolved iodine and potassium iodide—essentially potassium tri-iodide. Other solvents and phases may be used, however.
U.S. Pat. No. 5,350,644 to Graetzel, et al. discloses various dye cell chemistries, especially different dyes based on ruthenium complexes. Photons falling on the photoelectrode excite the dye (creating activated oxidized dye molecules), causing electrons to enter the conduction band of the titania and to flow (via an outer circuit having a load) to the counter-electrode. There, the electrons reduce tri-iodide to iodide in the electrolyte, and the iodide is oxidized by the activated dye at the photoanode back to tri-iodide, leaving behind a deactivated dye molecule ready for the next photon. U.S. Pat. No. 5,350,644 discloses that such dye cells can attain a solar-to-electric conversion efficiency of 10%.
The cells disclosed by U.S. Pat. No. 5,350,644 to Graetzel, et al. (see FIG. 1a), are based on two sheets of conductive glass sealed with organic adhesive at the edges (the conductive glass projects beyond the adhesive on each side, allowing for current takeoff). These cells operate at a voltage of about 650 mV and a current density of 15 mA/square cm under peak solar illumination, with the counter-electrode being the positive pole. It is asserted therein that since the materials and preparation methods are low cost and the titania layer can be prepared in large areas, such cells could potentially provide a good route to low-cost photovoltaic cells. It is further argued that there might be significant cost savings over classical single crystal or polycrystalline silicon cells and even more recent thin-film photovoltaic cells, since these are all high cost and rely on expensive and often environmentally problematic raw materials, together with complex, costly, semiconductor industry processing equipment and production techniques. These drawbacks include the use of vacuum deposition and laser methods, clean-room protocols, use of toxic hydrides such as silane, phosphine etc., as raw materials, and the use of toxic active-layer materials containing cadmium, selenium or tellurium.
The ohmic loss via the conductive glass coated with tin oxide is a major problem of such cells. The tin oxide coating is extremely thin, being limited in thickness usually to below one micron due to the need to maintain a high light transmittance through to the dye/titania layer of the photoanode. Moreover, tin oxide is only semi-conductive and is mechanically weak, such that the current takeoff is significantly limited by such a cell design.
A photovoltaic cell having electrically conducting coatings on spaced, glass support panes is disclosed by U.S. Pat. No. 6,462,266 to Kurth, which is incorporated by reference for all purposes as if fully set forth herein. As shown in FIG. 1b, a portion of a photovoltaic cell 1 is shown in a cross-section with two mutually distanced support panes 2 and 3, which in their border zones are held by a sealing system 4, which extends along the whole circumference. The inner surfaces of support panes 2 and 3 are coated each with a conductive layer 5, and 6, respectively. Layers 5 and 6 are formed by a suitable metal or metal oxide, in the present case, SnO2. On layers 5 and 6, an arrangement of parallel conductor leads 7 and 8 are provided, preferentially made from silver or a silver alloy, or from copper or a copper alloy. These conductor leads are coated each with an insulating coating 10, which insulates conductor leads 7 and 8 electrically towards the interior of the cell. Coating 10 consists of a glass free of heavy metals, which was applied as a glass flow on conductor leads 7 and 8. Onto conductor leads 7 and 8 insulated by the glass coating 10, a further electrically conductive layer 11 and 12 respectively, made from tin oxide or a similar material, can be applied in order to obtain a still higher yield of photovoltaic cell 1. In the border zones of seals 13 and 14, no electrically conductive layers 5 and 6 are provided, i.e., such layers have been eliminated from this zone using a sandblasting process. In this manner, possible short circuits via the seals 13 and/or 14 are avoided. Onto these two seals, a thin layer 15 of a low melt soldering tin is applied in such a manner that exterior weather influences also can not act onto photovoltaic cell 1.
The photovoltaic cell taught by U.S. Pat. No. 6,462,266 has reduced ohmic loss with respect to the cell disclosed by U.S. Pat. No. 5,350,644 to Graetzel, et al., because conductor leads 7 and 8 are good conductors (e.g., silver paste screen printed on and fired at 600° C.), and because the overall thickness of conducting materials has been increased. It must be emphasized that U.S. Pat. No. 6,462,266 teaches strips applied onto tin oxide coated glass and does not teach, nor fairly suggest, the application of conductive strips directly onto the glass (e.g., prior to the application of a tin oxide layer). Furthermore, the emphasis is on single cell construction with no advantage offered for construction of a multi-cell module. Most significantly, the periphery (i.e., the sides) of the cells is devoid of any current takeoff means, any current takeoff being made on the active sun-facing surface of the cell, resulting in a waste of available area. Consequently, the improvement in the cell performance is far from sufficient. U.S. Pat. No. 6,462,266 also emphasizes the application of two separate layers of tin oxide, as well as very high temperature processing that precludes plastic cells.
U.S. Patent Publication No. 20030108664 discloses a substrate with recessed conductors prepared from silver compositions. However, no means for protecting the conductors from corrosion are taught. More importantly, there is no specific means described for achieving the requisite current takeoff without wasting available area.
To date, there has been no real commercialization of photovoltaic dye cells, despite the great techno-economic potential thereof. The principal problems remaining include scale-up of cells to widths much above one centimeter—and areas much above 50 square cm—due to excessive ohmic losses from the poorly conducting tin oxide layers on the glass or plastic, long term stability of the dye, and difficulties of sealing the cells against long-term dryout and performance degradation. A further problem in prior art cells and modules has been excessive surface area wasted in seals and conducting paths on the sun-facing side of the cell or module. The active current-producing area in such cases is often less than 70% of the geometric area (footprint) of the cell or module, providing a poor effective efficiency from the available area.
There is therefore a recognized need for, and it would be highly advantageous to have, an electrochemical cell, powered by sunlight, that is simple, efficient and robust, and successfully addresses the manifest shortcomings of the prior art.